1. Field of the Invention
The present invention relates to a channel allocation method used for mobile type communication devices located in a system including a plurality of macros and microcells which commonly use the same frequency band, and more particularly, it relates to a channel allocation method that is capable of studying the characteristics of interference waves arrived from other cells (in a radio zone) and of searching channels according to a channel searching order that has been set and of selecting and allocating one of the channels.
2. Description of the Prior Art
There are conventional channel allocation methods for mobile type communication devices. For example, as shown in FIG. 1, the conventional technique disclosed in the following literature (1) uses a method for re-using a frequency band used by a macrocell station 2a by a microcell station 4a in a system that has already been used around United State of America. This method is referred to as "a reuse channel fixed allocation method".
(1) Y. Kinoshita et al., "HIGH DENSITY SPACE DIVISION MULTIPLE ACCESS" AND "DOUBLE REUSE OF FREQUENCY CHANNEL", Proceedings of the 3rd IEEE International Conference on Universal Personal Communications, PP.552-557, 1994).
As shown in the flow chart of FIG. 2, the conventional technique disclosed in the following literature (2) uses a method of monitoring channels just before the allocation of a channel by a microcell to be installed in an area having a strong interference in a Personal Handy-Phone System in use around Japan, and of allocating the channel after checking the existence of interferences. This method is called as "an interference avoidance channel allocation method".
(2) J. E. Padgett et al., "OVERVIEW OF WIRELESS PERSONAL COMMUNICATIONS", IEEE communications magazine, pp.28-41, January, 1995.
As shown in the flow chart of FIG.3, the conventional technique disclosed in the following literature (3) uses a method of setting a priority to each channel in relatively weak interference area in which a specified channel can be used constantly without any interference and of allocating or assigning a channel having a relatively lower interference while updating the priority of the channel when a channel is allocated or assigned to be used. This method is referenced to as "priority channel allocation method".
(3) H. Furukawa et al., "A MICROCELL OVERLAID WITH UMBRELLA CELL SYSTEM", Proceedings of the 44th IEEE vehicular technology Conference, pp.1455-1459, 1994.
As shown in the flow chart of FIGS. 4A and 4B, the conventional technique disclosed in the following literature (4) uses a method of eliminating the channel, in a channel allocation operation to select a channel to be used, in which an interference has happened once a relatively weak interference area in which a specified channel can be used constantly without causing of any interference. This method is referred to as "a channel allocation method in which any channel of the occurrence of interference is eliminated for channel allocation".
(4) J. Worsham et al., "A CELLULAR BAND PERSONAL COMMUNICATIONS SYSTEM, Proceedings of the 2nd IEEE International Conference on Universal Personal Communications, 1993.
As shown in the flow chart of FIG. 5, the conventional technique disclosed in the following literature (5) uses a method of allocating a channel as long as the number of interferences happening in this channel is not more than a predetermined number. This method is an improved method of the method disclosed in the literature (4) described above. This method is referred to as "a channel allocation method in which the interference in a channel is happened at more than predetermined times is eliminated for channel allocation".
(5) J. Avery et al., "A COMMON AIR INTERFACE FOR A CELLULAR AUXILIARY PERSONAL COMMUNICATIONS SERVICE", Proceedings of the 5th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, pp.665-669, 1994.
In the reuse channel fixed allocation method designated by the reference character (1) described above, as shown in FIG. 1, for example, a microcell installed in a building in a macrocell K uses a fixed frequency band allocated for a macrocell A which is adequately separated in distance from the microcell in order to avoid the occurrence of interference.
In the interference avoidance channel allocation method denoted by the reference character (2) described above, like the channel search/allocation processes shown in FIG. 2, first of all, a unused channel i is allocated (Step S12a) when a new channel allocation request is happened in a call/intracell hand-off operation (switching of a channel during calling). Then, the interference power of the channel i is measured (Step S13), and the measured power is compared with a threshold power level (Step S14). When U.sub.i &lt;r, (at Step S15, there is an interference wave), a next channel is selected when there is a channel that is not searched (Step S16), the processes described above are repeated. When there is no channel having the condition U.sub.i &lt;r after the search of all of channels are completed, the channel allocation operation enters a calling loss. At Step S14, when U.sub.i &lt;r, this channel is allocated (Step S18).
In the priority channel allocation method designated by the reference character (3), like the channel search/allocating processes as shown in FIG. 3, when a new channel allocation request is happens, first of all, the channel having the maximum priority level is selected (Step S12b). Then, the power level U.sub.i of the interference wave in this channel is measured (Step S13), and this power level U.sub.i is compared with a threshold power level r (Step S14). When U.sub.i .gtoreq.r, the priority level of this channel is decreased (Step S14a), and then it is checked whether or not the all of channels are searched (Step S15). When there is un-searched channel, the channel having the next priority level is selected (Step S16b), and the processes described above are repeated. If there is no channel having the condition U.sub.i &lt;r after all of the channels are searched, the new channel allocation request becomes a calling loss (Step S17). Thus, the priority level of the channel in which there are the interference waves is decreased by this method. In addition, at Step S14, when U.sub.i &lt;r, the priority level of the channel is increased (Step S14b), and the channel is allocated (Step S18). Thus, the priority level of the channel in which there is no interference wave is increased in this method. This method can allocate channels in which there is no interference wave because the channel search operation is performed from the channel having the highest priority level to the channel having the lowest priority level in order.
In the channel allocation method in which any channel of the occurrence of interference is eliminated for channel allocation designated by the reference character (4) described above, like an interference detection process shown in FIG. 4A and a channel search/allocation process shown in FIG. 4B, first of all, unused channel is selected (Step S2). Then, the interference power of this channel is measured (Step S5), and the measured interference power level U.sub.i is compared with an threshold power level r (Step S6). At Step S6, when U.sub.i .gtoreq.r, the channel i in which the interference wave is detected is eliminated from channel allocation (Step 6c) in order to avoid the occurrence of interference to a macrocell. When U.sub.i &lt;r, a next channel is selected (Step S9), and the processes described above are repeated.
When a new channel request has happened during the calling/intracell hand off operation, first of all, a channel i in the channels to be allocated is selected (Step S12c), then the interference power U.sub.i of the channel is measured (Step S13), and the measured interference power level U.sub.i is compared with an threshold power level r (Step S14). At Step S14, when U.sub.i .gtoreq.r, it is detected whether or not all of the channels are searched (Step S15). When there are un-searched channels to be allocated, one channel in the un-searched channels is selected (Step S16c), then the processes described above are repeated. When there is no channel having the condition U.sub.i &lt;r, this request enters the calling loss (Step S17).
When there is the channel having the condition of U.sub.i &lt;r (at Step S14), this channel is allocated (Step S18). In this method described above, the channel having the interference wave is eliminated from channel allocation. By using this method, no interference has happened in allocated channels.
In the channel allocation method in which the interference in a channel has happened at more than predetermined times is eliminated for channel allocation denoted by the reference character (5) described above, like an interference wave detection process shown in FIG. 5, first of all, one (a channel i) of unused channels is selected (Step S2), the interference power level U.sub.i of the channel i is measured many times at each time period (Step S5a). Then, this interference power measuring process is repeated for all of the channels and a time rate P.sub.i where the interference power level exceeds a threshold power level per time period in each channel is obtained (Step S6d, Step S9, and Step S6e). The channel whose time rate P.sub.i is not less than a predetermined time rate P.sub.t is eliminated for channel allocation (Step S6f). Thus, in this method, the channel is used for channel allocation so long as the number of interference occurrences in this channel is not more than predetermined time.
In the prior art, there are the conventional channel allocation methods (1) to (5) described above, we summarizes these methods as follows:
(1) The reuse channel fixed allocation method, as shown in FIG. 1, where selected or fixed channels are reused in the same frequency band. PA1 (2) The interference avoidance channel allocation method, as shown in FIG. 2, where the interference avoidance allocation operation is executed only during the channel allocation operation. PA1 (3) The priority channel allocation method, as shown in FIG. 3, where the priority level of a channel is updated only when the channel allocation is executed. PA1 (4) The channel allocation method in which any channel of the occurrence of interference is eliminated for channel allocation, as shown in FIGS. 4A and 4B. PA1 (5) The channel allocation method in which the interference in a channel is happened at more than predetermined times is eliminated for channel allocation, as shown in FIG. 5. PA1 (11) In the conventional channel allocation method (1), there is a possibility to cause an interference under a circumstance that it is easy to happen interference between microcells and macrocells when the microcells are installed at a higher floor in a building or when there are many existing macrocells to easily cause interferences. PA1 (12) In the conventional channel allocation method (2), it can not avoid the occurrence of interference during calling because the allocation avoidance operation for the channels that have had no interference is executed only during the channel allocation operation. Therefore an intracell handoff is required so as to increase the load of a microcell station and the number of interrupts of calling is increased, so that the quality of calling is decreased. PA1 (13) In the conventional channel allocation method (3), it can not avoid the occurrence of interference at the following channel allocation operation even if a channel priority level is high at the last channel allocation operation. In this case, the priority level must be decreased in this channel allocation operation. Therefore the channel priority level is only decreased and increased in this method, not constantly decreased. This method is used for a microcell that is installed at a lower floor (the first floor or the second floor) in a building in which the number of the occurrences of interferences to a macrocell is relatively low (relatively lower interference condition). However, by using this method, it is difficult to reduce the interference between macrocells and microcells installed in a higher floor in a building in which the number of the occurrences of interferences to the macrocells is high (higher interference condition). PA1 (14) In the conventional channel allocation method (4), it is difficult to use this channel allocation method in a circumstance where the number of the occurrences of interferences is high (the higher interference condition), because interferences are detected in all of channels and those channels are eliminated from the channel allocation operation, so that there is no channel to be allocated in the circumstance. PA1 (15) It is difficult to use the channel allocation method (5) in a circumstance where the number of the occurrences of interferences is higher (the higher interference condition), because there is the case that the number of channels, (each time rate indicating that the interference power is over a predetermined power level exceeds a predetermined time rate), to be allocated becomes zero.
However, there are drawbacks (11) to (15) in the above conventional channel allocation methods (1) to (5), as follows: